This invention relates to non-intrusive tap means for injecting light into, and extracting light from, the bound modes of a fibre consisting of a core whose refractive index is higher than that of the surrounding optically transparent cladding. Core and cladding are typically manufactured from silica based glasses, but other optically transparent materials may be used. The fibre may optionally be surrounded by a protective coating whose refractive index is greater than that of the optical cladding, and which does not absorb strongly at the wavelengths of interest. Examples of materials of such coatings include epoxy-acrylate, urethane, and some silicone resins. Fibres are conventionally cylindrically symmetric, but this is not essential for the purposes of this invention.
The present invention is preferably applied to fibres which are single moded or few moded at the wavelengths of interest. In multimode fibres the performance of taps based on bending or microbend induced coupling is sensitive to the initial modal power distribution between the bound modes of the fibre. As a result, the performance of such taps on multimode fibres may vary widely, depending on the power distribution launched into the fibre, the degree of mode coupling in the fibre, and the spacing between taps in a system in which more than one tap is applied. In single mode fibre such problems are avoided.
It is convenient to consider the extraction of light from the bound modes of the fibre in three stages as follows. First, the fibre is perturbed so as to induce coupling between one or more of the bound modes and one or more of the cladding modes. Second, the cladding power is transmitted across the boundary between the cladding and the protective coating or surrounding medium. Third, the radiated light is collected and focussed onto a suitable detector Light injection is achieved by a reciprocal process, i.e. all the rays may be reversed in direction, and the detector replaced by a light source. The principal difference is that if a coherent source such as a semiconductor laser is used for injection, it is advantageous to minimise the aberrations in the imaging optics as far as possible in order to maximise the power transferred to a particular mode. The sensitive area of a semiconductor detector will typically be much larger than the emitting area of a laser, and good collection efficiency is achievable with less highly corrected optics.
Coupling between the bound and cladding modes is achieved by applying a suitable perturbation to the fibre. For coupling between the fundamental LP.sub.01 mode and a higher order mode with azimuthal mode number unity, this is achieved most conveniently by a periodic bending of the fibre core The optimum bending pitch for resonant coupling is inversely related to the difference in propagation constants, or axial components of the wavevector, for the two modes, and should be close to the period of beats between the two modes, as described in UK Patent Application GB 2182516A. The pitch can be calculated to acceptable accuracy by solving the scalar wave equation for the two waveguide modes at the wavelengths of interest, or by measuring the attenuation induced in the fibre by periodic bends of known pitch.
The bend amplitude needed to transfer a useful fraction of the available power by resonant coupling between two modes is very small, typically a fraction of one micrometre for a periodic bend of pitch 0.2 to 1.0mm applied over a few millimetres length of the fibre.
In situations where the optical cladding is contacted by a higher refractive index medium, such as a coating, the cladding modes to which power is coupled from the bound modes are not true eigenmodes of the core/cladding/coating structure because power is lost at each reflection at the cladding/coating boundary. For typical telecommunications fibres, the wavevector of the lowest order cladding modes that are most readily excited are nearly parallel to the fibre axis. Consequently, the Fresnel reflection coefficient at the boundary with the coating is almost unity, and power may propagate for several centimetres or more with relatively low losses if the fibre is straight Low losses enhance the resonant coupling of power between the bound and cladding modes, but inhibit the transfer of power across the cladding/coating boundary.
This problem can be somewhat alleviated by applying a periodic perturbation of higher spatial frequency than is required for resonant coupling to low order cladding modes so that higher order cladding modes may be selectively excited, or coupled into the fundamental bound mode. This may be achieved by pressing a periodic undulation or grating of shorter pitch against the fibre. The higher order modes propagate at a greater angle to the interface, and so have a lower Fresnel reflection coefficient, and higher transmission coefficient.
From the foregoing it will be evident that the light emanating from the fibre as the result of the deliberately induced microbending emerges over an area with a relatively high aspect ratio which typically measures several millimetres in length but not more than about 0.1 to 0.5mm in width. For light coupled from the fundamental mode into a specific cladding mode, this light will emerge at a specific angle to the fibre to emerge within a fairly narrow range of angles to the fibre axis, and with the azimuthal distribution of that light being dependent upon the geometry of the microbending. To detect that light efficiently requires an optical system which will collect a high proportion of the emanating light from this high aspect ratio source, and focus it on a detector to an image having a size and shape matched with that of the photosensitive surface of that detector. A similar optical system is similarly required for coupling light from a source efficiently into the fibre, though in this instance rather better quality of optical system may be necessary because though, on the extraction side, it is generally not difficult to accommodate optical imaging aberrations by choice of a detector with a larger photosensitive area; it is not so easy on the insertion side to accommodate aberrations by changing to a smaller area source.
In U.S. Pat. No. 4 253 727, to which attention is directed, there is described a coupling device in which a multimode optical fibre is clamped between two transversely corrugated parts of the device in order to induce mode coupling and consequent leakage of light propagating in the fibre into the two part device. It is explained that this configuration is in principal able to collect and collimate, by means of a conical reflecting or refracting surface all light launched from the fibre into the device at an angle .theta. to the unperturbed fibre axis over the whole range of azimuth angle .phi. from 0.degree. to 360.degree.